Field of the Invention
The invention relates to an antitheft system for a motor vehicle in which locking or unlocking of one or more doors is made possible only after authorization is proven.
In antitheft systems for motor vehicles, in particular in the case of keyless access control systems, a request signal is transmitted to a portable code transmitter after triggering through the use of an activation switch on a door. The code transmitter responds automatically with its response signal if it has received the request signal. If the response signal itself is received in the motor vehicle and if it proves to be authorized after comparison with a reference signal expected there, the doors of the motor vehicle are locked or unlocked.
In an antitheft system which is known from German Published, Non-Prosecuted Patent Application DE 41 23 654 A1, a switch in the interior is activated when the occupants leave the motor vehicle. As a result, a request signal which firstly addresses the code transmitter in the interior is triggered.
If no signal is sent back by the code transmitter from the interior, the request signal is transmitted to the exterior. If a response signal is transmitted back from there, the vehicle can be locked, since it is assumed that the code transmitter is outside the motor vehicle. However, if the user with the code transmitter moves away from the vehicle too quickly, the motor vehicle may possibly no longer receive the response signal. The vehicle then remains unlocked.
In a further antitheft system which is known from U.S. Pat. No. 4,805,723, the doors of the motor vehicle are automatically locked if the doors are closed and no one is subsequently detected in the motor vehicle. However, in that context it may be the case that the driver has forgotten his or her key in the motor vehicle and can then no longer enter his or her motor vehicle again since it is already locked.